Broadband Access Networks Network capacity is not always
Broadband Access Networks Code division multiplexing (CDM) uses complex coding to separate multiple channels. CDM is a spread-spectrum technique. In this approach, each user s communication is spread across the available spectrum along with everyone else s but is encoded in such a way as to enable separation of different communication channels. This approach to multiplexing is widely used in wireless communication since it tends to be more resistant to the random variations in signal strength that wireless transmission is commonly subjected to and it makes very efficient use of scarce wireless bandwidth. 8.2.2 Data Link Layer The data link layer is concerned with transmitting blocks of bits, called frames, from the transmitter to the receiver. In cases where multiple users access a single physical channel, it will usually provide some mechanism for managing the resulting contention which may occur if multiple users attempt to access the medium at the same time. This mechanism is identified as belonging to a sublayer within the data link layer, called the Medium Access Control (MAC) layer. The data link layer may also be responsible for error detection and correction and perhaps include some mechanism for retransmission of corrupted or missing frames. Most broadband access networks, with the exception of ADSL use a shared medium and hence have an MAC sublayer. The most commonly used techniques for sharing access are contention-based schemes, where devices wishing to access the medium wait for it to be idle for a period of time before transmitting. If other users also transmit during this time, causing a collision, there must be some mechanism for resolving the conflict and retransmitting. Usually the approach is for all users to wait a random amount of time before retransmitting once the shared medium has become silent. This approach is used in some wired networks such as Ethernet and also some wireless networks such as WLAN. Another approach is a master-slave polling system where one device or node controls access to the medium. It specifies which device may transmit. This approach is also used in some wireless networks, notably Bluetooth and less commonly, WLANs. The data link layer may also include some form of error correction or error detection, but this too is strongly dependent on the physical medium. In very reliable, high capacity media such as fibre optics, it may be unnecessary to provide any error correction. Where the medium is inherently unreliable, such as wireless, some form of error correction or detection is essential. The physical and data link layers characterise the Access Network. They specify the medium to be used, how information is encoded, how users are multiplexed onto the medium and if any error control or detection is to be used. We now look at the most significant broadband access networks. 8.3 Cable Networks Cable television networks are commonly used as an access network where there is a substantial cable television infrastructure. Using cable television networks as an access network allows owners of cable television networks to leverage their investment by providing network access to business and residential customers [ARM2000]. Cable television networks are usually implemented as an inverted tree, with the root of the tree referred to as the head-end where programming is injected into the network. The head-end supplies programming to the branches of the tree, which correspond to delivery
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